1. Field of the Invention
The present invention relates to a leadframe used for a semiconductor device and, more particularly, to a leadframe for a semiconductor device which shows improved adhesion with sealing resin as well as improved solder wettability.
2. Description of the Related Art
A leadframe for a semiconductor device is used for mounting a semiconductor device, which is fabricated by sealing a semiconductor chip with resin material so as to be integrated with the leadframe, on a substrate or the like. In general, the leadframe has a stage section where a semiconductor chip is mounted; an inner lead section which is connected to the stage section and electrically connected to electrodes of the semiconductor chip through wire bonding; and an outer lead section, or the like, which is connected to the inner lead section and acts as an external connection terminal when the semiconductor device is mounted on the substrate, or the like. Such a leadframe is required for adhesion with a resin material used for sealing the chip. Also required is a superior bondability of an outer lead which is used for bonding the leadframe to a substrate so that a semiconductor device is mounted thereon by means of soldering or the like.
FIG. 6 is a plan view showing an example leadframe for a semiconductor device. In a leadframe 10, reference numeral 12 designates outer leads; 14 designates inner leads; 16 designates a stage section forming a chip mount section where a semiconductor chip (not shown) is mounted and which is connected to rails 20, 20 by means of a support bar 18. Reference numeral 22 designates a dam bar.
The semiconductor chip (not shown) is mounted on the stage section 16 of the leadframe 10. This semiconductor chip and the inner leads 14 are bonded by means of wires. The semiconductor chip, the wires, and the inner leads 14, all of which are situated within an area encircled by a broken line shown in FIG. 6, are sealed with resin, to thus complete a semiconductor device. In order to mount the semiconductor device on a substrate, or the like, soldering is usually utilized. In recent years, a leadframe, where a coating film has been formed over the outer leads 12 in advance (also called “exterior solder coating film”), is frequently utilized.
A leadframe enabling mounting a semiconductor device on a substrate without involvement of an exterior solder coating film had been known, which is comprised of a substrate for use with a leadframe, generally being called as a Pd-PPF (Palladium Pre-Plated Leadframe), a nickel (Ni) plating layer of a ground layer, an intermediate layer of a palladium (Pd) or Pd alloy coating film, and a surface layer of gold (Au) plating film or silver (Ag) coating film, those being sequentially formed on the substrate as disclosed in JP-A Hei 4-115558.
A leadframe, which is disclosed in JP-A Hei 4-337657 as another related-art example of a leadframe for a semiconductor device having a leadframe substrate being coated with exterior plating of material other than solder, is comprised of: an Ni-based plating layer provided on a base material of the leadframe; a Pd or Pd alloy plating layer provided on at least inner lead sections and outer lead sections on the base material; and an Au plating layer provided on the Pd or Pd alloy plating layer. Further, JP-A Hei 11-111909 also discloses a leadframe coated with substantially similar plating. Still Further, JP-A 2001-110971 discloses a leadframe having a Ni-based protective coating layer provided on a base material of the leadframe, an intermediate layer of Pd or Pd alloy plating, and an outermost layer formed by plating the intermediate layer with Pd and Au one after another.
Meanwhile, in recent years, a lead (Pb)-free soldering material has come into general use as a soldering material used for mounting a semiconductor device on a substrate, or the like, in view of environmental protection. Tin-zinc (Sn—Zn)-based solder, tin-silver-copper (Sn—Ag—Cu)-based solder, or the like, has been put into practical use as such a lead-free soldering material.
A related-art leadframe (a so-called Au/Pd/Ni leadframe) is formed by sequentially forming, on a base material for a leadframe, an Ni plating layer, a Pd or Pd alloy plating layer, and an Au plating layer. A semiconductor chip is mounted on the leadframe and sealed with a resin material, to thus produce a semiconductor device. When the semiconductor device is mounted through use of lead-free solder in place of conventionally-widely-used tin-lead solder, the fusing point of the employed lead-free solder is higher than that of the related-art tin-lead solder. For this reason, a reflow temperature must be increased. For instance, the fusing point of Sn—Ag—Cu solder, which has recently come to be used, is 217° C., and a temperature of about 240° to 250° is used for reflowing of the solder.
When the reflow temperature has increased, sealing resin is easily exfoliated from the leadframe because of a difference in coefficient of thermal expansion between a metallic material of the leadframe base material and the sealing resin. Because of contribution of hygroscopicity of epoxy-based resin, which is commonly used as sealing resin, moisture easily intrudes into cracks caused by exfoliation. The moisture is vaporized in subsequent thermal treatment, or the like, which in turn becomes a cause of serious defects, such as cracks in the sealing resin, fractures of the semiconductor chip, or the like.